To properly understand and interpret the mineralogy of a sample, knowledge of both the textural information of the sample and the spatial distribution of minerals in the sample is required.
The textural information of a geological sample refers to the size, shape and arrangement of grains or crystals within the sample, the homogeneity of the sample, its degree of isotropy, and the distribution of elements within the minerals The textural information can be measured, for example, by scanning the sample with a high energy beam of charged particles, and measuring at each scanned position the intensity of back-scattered electrons (“BSE's”) that are received at an electron detector. The measured BSE intensities are indicators of the average atomic number of elements in the sample as a function of scan position. A plot of the intensity versus scan position can be presented as a grey scale image, where each pixel in the image is a shade of grey that corresponds to the measured BSE intensity at the scanned position. The image so obtained will be darkest where the measured BSE intensity is least, and lightest where the measured BSE intensity is greatest. Of course, this color scheme can be inverted so that the image will be lightest where the measured BSE intensity is least, and darkest where the measured BSE intensity is greatest.
In addition to BSE intensity, other forms of textural information can include representation of the rate of x-ray production at each pixel. This is measured by scanning the sample with a high energy beam of charged particles, and measuring the intensity of x-rays produced by the sample and received by an x-ray detector. The x-rays are produced when the charged particles dislodge electrons in the sample, and electrons from higher energy shells fall into the energy shells vacated by the dislodged electrons. The rate at which the x-rays are produced is roughly proportional to the volume and atomic number of the material in the sample. A plot of the x-ray generation rate versus scan position can be presented as a grey scale image, where each pixel in the image is a shade of grey corresponding to the measured rate of x-ray production at each scan position. The image so obtained will be darkest where the measured x-ray production rate is lowest, and brightest where the measured x-ray production rate is highest.
The spatial distribution of minerals in the sample is a plot of the minerals identified in the sample as a function of sample position. It can be generated by scanning the sample with a high energy beam, and measuring the energy distribution of x-rays emitted from the sample as a function of scan position. On a per pixel basis, these energy distributions can be fitted and/or compared to a catalog of energy distributions obtained from pure elements or pure minerals in order to identify the elements and/or minerals in the sample at each scanned position. Techniques for identifying minerals based on a catalog of elemental x-ray spectra are disclosed, for example, in U.S. patent application Ser. No. 12/866,697, filed on Feb. 6, 2009, which is herein incorporated by reference in its entirety. Techniques for identifying minerals based on a catalog of mineral x-ray spectra are disclosed, for example, in U.S. patent application Ser. No. 14/073,523, filed on Nov. 6, 2013, which is herein incorporated by reference in its entirety. Different colors can be assigned to different minerals in the catalog, and an image of the spatial mineral distribution in the sample can be generated by plotting the colors of identified minerals as a function of scanned position.
Since proper understanding of the mineralogy of a sample requires knowledge of both the textural information of the sample and the spatial distribution of minerals in the sample, methods are needed to combine the textural and spatial mineral distribution images into a single composite image that reveals both the spatial mineral distribution and textural information about the sample.